This invention relates to an inner shroud for the stator blades of a gas turbine, in particular an aircraft turbine, which is provided with means for the rotatable support of the stator blades and for the provision of a seal between the rotor stages and against a secondary air system.
On a gas turbine, for example the compressor arranged upstream of the combustion chamber, one stator blade row each is arranged between adjacent rotor stages. The forward stator blade stages are variable. The variable stator blades are supported on spigots in the compressor casing. The inner shroud is arranged at the inner spigot of the stator blades. The variable stator blades serve to guide the air at an optimal angle into the subsequent rotor stage.
The circular inner shroud for the stator blade row is usually made of a steel or aluminum alloy and normally includes two semi-circular ring segments which may also be split in the axial direction, in which case they are secured to each other by suitable fasteners upon assembly. One main function of the inner shroud is to rotatably support the individual stator blade by means of a spigot which is connected to the stator blade and located in a plain bearing bushing fixed in the inner shroud. As the inner shroud is made of a metallic material, the required mechanical properties, such as stiffness and strength, and a high resistance against erosion or foreign object impact are ensured. In a further main function, the inner shroud provides a seal against the rotor to prevent compressed air from recirculating from areas with higher pressure into areas with lower pressure. For this purpose, sealing fins are provided on the rotor opposite of an abradable sealing ring which is separately fitted to the inner shroud supporting the stator blades. Finally, in still another main function, the inner shroud provides a seal against the secondary air system. This is effected, for example, by the interaction of a static seal and a mating sealing face of the inner shroud.
While the inner shrouds of the type specified above satisfy the requirements for strength and heat expansion, they are disadvantageous in that their manufacture by forging and subsequent machining is time and cost-consuming and, further, in that the inner shroud has high weight. In addition, the manufacturing effort is very high, due to the fact that the plain bearing bushes and the seals, as separately manufactured components, are costly and must be fitted or assembled individually.
In order to avoid the disadvantages resulting from high weight, Patent Specification U.S. Pat. No. 5,062,767 already proposes an inner shroud for the stator blade row which includes a C-shaped steel ring into which abutting segments made of a ceramic material are inserted. The spigots of the stator blades are located in the ceramic segments, these segments having low weight, good strength and sliding properties and being temperature and erosion-resistant. The arrangement of the ceramic segments in the steel ring ensures that the inner shroud has thermal expansion characteristics which, all in all, harmonize with the prevailing temperatures and the thermal expansion of the other components of the compressor. While this design provides for weight reduction and the required thermal expansion, it is again disadvantageous in that the manufacturing effort for the inner shroud of the metallic ring, the ceramic segments and the separate sealing elements, is still high.